1. Field of the Invention
The present invention generally relates to noise countermeasure determination methods and apparatuses and storage media, and more particularly to a noise countermeasure determination method which determines noise countermeasures for guaranteeing a normal operation of an electronic circuit by minimizing noise which may be generated in the electronic circuit when designing the electronic circuit such as a large scale integrated (LSI) circuit, multi-chip module (MCM) and printed circuit board (PCB), and to a noise countermeasure determination apparatus which determines the noise countermeasures by such a noise countermeasure determination method, and to a computer-readable storage medium which stores a program for causing a computer to carry out such a noise countermeasure determination.
Recently, the noise countermeasures and noise analysis have become important when designing the electronic circuit, particularly because the size of electronic circuits has decreased considerably and the operation speed of the electronic circuits has increased greatly. The noise countermeasures refer to various measures which are taken to suppress the noise generated in the electronic circuit, based on results obtained by the noise analysis.
2. Description of the Related Art
Conventionally, various kinds of noise analyzing tools have been proposed to carry out the noise analysis when designing the electronic circuit. The noise analyzing tool carries out the noise analysis and a noise check using a circuit simulator after a layout design of the electronic circuit is made, so as to determine noise countermeasures for suppressing the noise. The design of the electronic circuit is modified if necessary based on the determined noise countermeasures. After such a design modification, the noise analysis and the noise check are carried out again, and the above described procedure is repeated until the noise falls within a tolerable range.
The noise which is to be mainly considered when designing the electronic circuit includes reflection noise and crosstalk noise. Normally, the reflection noise is generated by a mismatch of an internal resistance of a driver element and a characteristic impedance of a transmission line. In order to suppress the reflection noise, a method has been proposed to insert a damping resistor in series to an output of the driver element particularly in the case of a 1:1 transmission. According to this proposed method, a resistance of the damping resistor is selected so that a sum of the internal resistance of the driver element and the damping resistance becomes equal to the characteristic impedance of the transmission line.
The reflection noise in transmissions other than the 1:1 transmission, such as a 1:N transmission, greatly depends on the wiring (or routing) topology. For this reason, the wiring topology is selected manually, so as to carry out the wiring to suit the wiring topology selected by the designer. Accordingly, the circuit simulator carries out the noise analysis and the noise check based on the wiring information. If noise exceeding a tolerable range exists as a result of the noise analysis, operations such as a modification of the wiring topology, a rewiring to suit the wiring topology, a noise analysis and a noise check are repeated so as to find an optimum wiring topology.
On the other hand, the crosstalk noise greatly depends on the driven ability of the driver element, a gap between adjacent patterns, and the like. Usually, the crosstalk analysis requires information related to the adjacent patterns, and for this reason, the noise analysis and the noise check are carried out after the layout design of the electronic circuit is made, using layout design data. The information related to the adjacent patterns include a pattern gap, a distance for which the patterns run parallel to each other, a position where the patterns are parallel on the transmission lines, and the like.
However, due to the further size reduction and the further increased operation speed of recent electronic circuits, the number of nets which require the noise analysis and the noise check is increasing, thereby increasing the number of design steps. A net refers to a part which is made up of at least one target circuit element when designing the electronic circuit. For this reason, it is necessary to take noise countermeasures not requiring a backward process which is a repetition of manual operations including circuit design, layout design and noise analysis. In other words, prior to the circuit design and the layout design, it is necessary to create a circuit model amounting to at least one net, so as to determine the noise countermeasures based on the circuit model input.
But even in a case where the circuit model amounting to at least one net is created and the noise analysis and the noise countermeasures are carried out, before the circuit design and the layout design, a processing time as a whole becomes considerably long if the noise countermeasures are always determined by the circuit simulator, since a processing time of the circuit simulator is long compared to other processes. Consequently, the problem of the considerably long processing time of the circuit simulator becomes particularly conspicuous when repeating operation cycles such as design, analysis, countermeasures (design modification) and analysis. For this reason, there were demands to determine the noise countermeasures by suppressing the operation of the circuit simulator to a minimum.
In order to suppress the operation of the circuit simulator to a minimum, it is necessary to select the damping resistance without using the results of the circuit simulator, even for the countermeasures against the reflection noise. However, if the damping resistance which matches the output resistance of the driver element and the characteristic impedance of the wiring is used as the recommended circuit information, this damping resistance will not match the damping resistance which is already inserted in the input circuit information, even though a problem will not occur in the actual transmission waveform. As a result, there was a problem in that the noise countermeasures which are determined in such a case may be too severe and require the damping resistor to be modified.
In addition, when determining the wiring topology, it is necessary to repeat the wiring topology selection, wiring modification and noise analysis. But if the wiring topology selection and the wiring modification are to be made by the designer, there was a problem in that it takes too long to carry out such operations manually. For this reason, there were demands to repeat the wiring topology selection, wiring modification and noise analysis within a short time, and to determine the noise countermeasures by selecting an optimum wiring topology.
Furthermore, according to the crosstalk noise analyzing method described above, the noise analysis and the noise check are made using the design data after the layout design. Accordingly, when the crosstalk noise is found as a result of the noise check, the layout design must be modified with a backward process which repeats manual operations. Consequently, there were demands to reduce the design steps by eliminating the backward process which repeat manual operations.